Propellant gases are required to have certain properties:
they must be gaseous in the anticipated range of temperatures in which they are to be used, particularly around room temperature, and they must be liquid under a pressure which permits the use of lightweight disposable containers, i.e. the pressure required is insufficient to cause the container to burst at the highest temperature likely to be experienced by the container in normal use. They must also be economic to use. As a result, amongst the most commonly used propellants today are propane, n-butane, i-butane, dimethyl ether, methyl ethyl ether, HFA 134a (1,1,1,2-tetrafluoroethane), and HFA 227 (1,1,1,2,3,3,3-heptafluoropropane), the latter two having particular application in the medical field for aerosol inhalers. In the present specification, when not otherwise stated, “butane” refers to either isomer, or any mixture of both isomers of butane.
Under current regulations, every single propellant containing container, such as aerosols or propane and/or butane fuel Containers (which Contain propane and/or butane as both product and propellant), must be tested for leakage during production. The standard method is based on a hot water bath, as described in the aerosol dispensers directive 2008/47/EC. This process is extremely costly in terms of equipment, energy, and labour, since it requires a large water bath (±/−20,000 L) to be maintained at a temperature of 50° C., and an operator must watch for bubble formation and manually reject any leaking containers. Each operator is limited to a testing rate of about 60 units per minute, and the possibility for human error is high. In addition, a large amount of waste water contaminated with leaking products from the aerosol cans is produced.
Several alternatives to the water bath method have been developed.
The first of these is a detection method based on pressure change detection, developed by the Applicant, for leak testing the crimp and valve of filled aerosol cans. This system was designed to meet the specifications given in UN/SCETDC/INF.93, that is to say it is capable of detecting leak rates of 2.0×10−3 mbar 1 s−1. Cans are placed onto a continuously moving carousel, and test chambers are then lowered over the top of the aerosol cans, hermetically sealing the crimp and valve section. Subsequently, a slight overpressure is created in the test chamber, and any leakage of the can causes a pressure change in the test chamber which is then detected. However, current versions of the system are limited in throughput since the pressure change detection is relatively slow, thus multiple units in parallel are required to achieve high production speed, which is commonly demanded at the 500 per minute level. This level of throughput requires approximately 60 test chambers.
A second alternative to the water bath is based on the principle of flame ionisation detection. This method is however slow, and the testing rate is currently limited to approximately 50-100 containers per minute, requiring between 5 and 10 costly detectors in parallel to achieve a 500 per minute test rate. In addition, due to safety and cost considerations, the presence of a flame in the context of propane/butane leak detection is undesirable.
A third alternative to the water bath is provided by an optical test method based on quantum cascade lasers in the 7.2 μm frequency range. The lasers are arranged to chirp so as to provide a frequency-range scan, and the received signal is subject to digitised spectrum analysis to identify the spectroscopic peaks in a sample captured from around an aerosol can. To do this, a processor runs a fitting algorithm over the spectrum produced by the sample of gas that has been captured to determine whether or not butane or propane is present. However, since this frequency analysis is carried out in the 7.2 μm range, the propellant absorption spectra overlap that of water, which reduces the precision of the analysis and requires significant computing power to perform, since spectral analysis of the received laser light is a computationally-heavy method per se.